Hi Mushi,
There's really two different things to be discussed here; the first is the actual exposure captured by the camera, and the second is the adjustment of the apparent exposure during post processing.
Actual exposure captured by the camera:
Often the camera can do an excellent job of working out what that exposure should be, but often also, it needs a helping hand to shift the exposure up or down a little. I won't go into extreme detail just yet because this post would end up being a full chapter long, but in the case of the shot you took, you'll see a large triangular area on the right hand side (camera right) of the young lady'sd nose that - although not (quite) blown out - none-the-less lacks any detail. In other words you have almost white skin in an area where the skin should look more tanned.
Adjustment of apparent exposure:
When adjusting your image I brought the exposure back a full stop, but even that didn't entirely fix the problem area -- so I blended in some skin tone from her forehead. It's still not a perfect result, but I think it looks a lot better.
I suspect that what's happened here is that the light coming through the window to the lady's right has reflected off the skin in that area, and bounced back towards the camera giving the large hot spot. The way to have cured that at the time you took the shot would have been to use a diffuser to greatly soften the light ... then the rest of the capture would have fallen into place quite nicely on this occasion.
So there's your first task - go buy a diffuser; they're pretty cheap and they can make a BIG difference to the shot. If you have someone who can hold it for you then that's all you need to start with -- if you don't then eventually you'll need to get a holder + boom arm + stand to hold it.
Here's a shot of how I used a diffuser for one of the shots above: (Click for larger view - for some reason it's only showing in a small size)
original.jpg